The study is designed to investigate the role of the gastro-intestinal tract as an endocrine gland in the maintenance of glucose homeostasis. Among the identified gut hormones, gastric inhibitory polypeptide (GIP) appears to be the major insulin secretogogue. The glucose clamp technique has demonstrated the role of the endogenously-released hormone under in vivo conditions in normal man. GIP infusion studies have shown that the hormone has the same effects when exogenously administered. Furthermore, jejunoileal bypass for morbid obesity results in marked reduction in insulin response to an oral amino acid load.